From prior German Patent, DE 44 36 977.8 there is known an apparatus for the electronic surveillance of articles protected by resonant circuits. To increase the sensitivity relative to interference while on the other hand obtaining a high detection rate, both the amplitudes of the received signals and the phase differences between the transmitter field and the received signals are evaluated. The resonant frequency of the security elements varies on account of manufacturing tolerances. In order to ensure that all security elements are detected within predetermined tolerances, the transmitting device cyclically emits into the interrogation zone an interrogation signal of a bandwidth tuned to the tolerances specified in the manufacture of the security elements. The comparison values used are predetermined threshold values or previously stored curve patterns. From this the disadvantage of this prior known apparatus results: The actual sources of interference acting on the received signals in or in the vicinity of the interrogation zone are not considered or only insufficiently considered.
To detect the presence of electromagnetic security elements in an interrogation zone, it is proposed in European Patent EP 123 586 B to emit into the interrogation zone, in addition to two interrogation fields with the frequencies F1 and F2 in the kilohertz range, a field with a frequency F3 in the hertz range. The two interrogation fields with frequencies F1 and F2 cause a security element present in the interrogation zone to emit a characteristic signal with the intermodulation frequencies n.multidot.F1.+-.m.multidot.F2 (where n, m=0, 1, 2, . . . ). The low-frequency interrogation field causes the security element to be driven from saturation in one direction into saturation in the other direction at the clock rate of this particular field. As a result, the characteristic signal occurs periodically at the frequency of the low-frequency field.
As an alternative solution, it has further become known to use only one interrogation field in the kilohertz range for excitation of the security element, with the characteristic signal of the security element occurring again at the clock rate of a low-frequency field cycling the magnetically soft, non-linear material between the two states of saturation.